The present invention relates to a mechanical igniter for a gas generator that supplies a working gas to a pretensioner in a seat belt system or to an air bag in an air bag system. More particularly, the present invention relates to a safety device used to prevent an erroneous actuation of the mechanical igniter.
Vehicles, for example, automobiles, are often provided with a seat belt system or an air bag system or both. For a pretensioner in the seat belt system or an air bag in the air bag system, a gas generator is employed as a device for generating a driving medium for the pretensioner or the air bag. The gas generator is activated by a combination of an acceleration sensor and an igniter which is activated in response to a signal from the acceleration sensor. In general, an electric acceleration sensor and an electric igniter are used for this purpose. Therefore, the sensor requires a power supply, and electrical conductors are needed to conduct firing signals from the sensor to the igniter because the sensor is located in the front part of the vehicle, which is suitable for sensing acceleration, whereas the igniter is attached to the gas generator on the pretensioner or the air bag. The costs for components and the labor and time required for assembly and installation of electrical sensors have resulted in proposals for a low cost mechanical igniter in which neither a power supply nor electrical wiring is needed and a sensor itself constitutes an igniter.
One such mechanical igniter, as disclosed in U.S. Pat. No. 4,955,638, has a weight, a trigger lever and a firing pin. When a predetermined level of acceleration acts on the igniter, the trigger lever is activated by inertial movement of the weight, causing the spring-loaded firing pin to perform a percussive action, thereby firing a fuse of a gas generator. The igniter has a safety device for preventing an unintentional firing. The safety device has a shutter plate constituting a restraint member which is reciprocatingly movable between a position in which it prevents the firing pin from performing a percussive action and a position in which it allows a percussive action of the firing pin. The shutter plate is linked to and actuated by rotation of a set lever that functions as a controller for the safety device.
The provisions for preventing unintended firing of the above-described mechanical igniter further include a multiple erroneous actuation preventing device including a rotary shaft that restrains the weight and the firing pin and a device that holds the seat belt by a plunger interlocked with the shutter plate. Moreover, measures are taken to prevent accidental movement of the set lever. It is therefore considered that the degree of reliability of the safety device is extremely high. On the other hand, the safety device is difficult to operate because the measures taken to prevent accidental movement of the lever are very strict.
Although it is important to take all possible measures to ensure reliability of the operation of the safety device with a view to preventing an erroneous actuation of the mechanical igniter, it is also important to facilitate the manner in which the safety device is released. The requirement for ensuring reliability and the requirement for ease of operation conflict with each other, and it is not easy to meet them both simultaneously.